Microelectromechanical systems (MEMS) are devices with characteristic dimensions of hundreds of microns or less, which integrate electrical and mechanical elements through microfabrication technologies. Micromirror devices are a type of microelectromechanical systems. A typical micromirror device, such as a device for use in spatial light modulation or a device in optical signal switching has an electrode, a deflectable and reflective mirror plate, a deformable hinge, and other desired functional members on a substrate.
Deflection of the mirror plate is enabled by attaching the mirror plate to the deformable hinge; and elevating the deformable hinge above the substrate using one or more hinge posts such that at least a portion of the hinge is free to be deformed. Deflection of the mirror plate can be controlled by the electrode disposed proximate to the mirror plate such that an electrostatic field can be established between the electrode and the mirror plate. Such electrostatic field yields an electrostatic torque to the mirror plate; and thus moves the mirror plate with the electrostatic torque.
As a way of example, FIG. 1A illustrates a micromirror device in the art. The micromirror has mirror plate portion 102 that further includes reflective mirror plate 110 and a mirror post (not shown) provided for attaching the mirror plate to the deformable hinge (112). Hinge structure portion 104 comprises deformable hinge 112, hinge arm 114, and posts (e.g. post 118) that are provided for attaching the hinge structure portion to the underneath electrode portion 106. The hinge structure portion (104) in this example has other features, such as elevated electrode 116 and landing tip 120. The elevated electrode (116) is for moving the mirror plate; and the landing tip is a flexible beam structure for limiting rotation of the mirror plate at desired rotational angles.
The electrode portion (106) has addressing electrodes, such as electrode 122 and a base to which the above hinge structure portion can be attached.
The mirror plate portion (102), hinge structure portion (104), and electrode portion 106 are assembled to substrate 124 having formed thereon electrical circuitry for controlling the micromirror.
To enable establishment of the electrostatic field, the mirror plate is desired to be electrical connected to the external sources, such as power sources and signal sources. An approach of such electrical connection is accomplished by electrically connecting the mirror plate to the hinge; and electrically connecting the hinge to the electrical contact pads that are often formed on the substrate. Electrical connection of the hinge to the contact pads are often accomplished by providing an electrically conductive layer inside the hinge post(s) and connecting such conductive layer to the deformable hinge and the contact pads. FIG. 1B illustrates an exemplary electrical connection of the deformable hinge in the micromirror device as illustrated in FIG. 1A.
Referring to FIG. 1B, electrode layer 136 is a metallic layer for electrode 122 in FIG. 1A; and electrically conductive layer 134 (e.g. a TiN layer) is formed on metallic layer 136. Electrical insulating layer 133, such as a SiO2 layer is formed on the conductive layer 134 to protect underneath conductive layer and prevent electrical short. Hinge layer 137, which is a member of the deformable hinge (112 in FIG. 1A), is connected to the conductive layer 134 through post 135 such that the deformable hinge is electrically connected to the underneath electrode structure.
For improving the mechanical property of the post, as well as the deformable hinge, internal sidewall of the post (135) is often covered by a mechanical enhancing material (113), which is a dielectric material, such as oxide material (e.g. SiO2).
Micromirrors with such electrical connection mechanisms, however, have disadvantages. For example, because the quality and performance of the electrical connection increases with increase of the dimension of the hinge post and the thickness of the electrical conductive layer of the deformable hinge, it is desired for a thick conductive layer in the deformable hinge and large hinge post. However, thicker deformable hinge and larger hinge post increase the size of the micromirror, which in turn limit size-reduction of the micromirrors. Moreover, the deformable hinge with increased dimension limits the possibility of operating the micromirrors with low voltages.
The oxide coatings on the internal sidewall of the hinge post also have disadvantages. For example, a process of forming such sidewall coatings during fabrication may generate defects in the micromirrors, which may reduce yield.